Photon entanglement router

ABSTRACT

A photon entanglement router comprises a modified birefringent spectral filter followed by a polarization beam splitter (PBS). Frequency degenerate or non-degenerate entangled photons, generated by a collinear laser source and incident on one input port of the photon entanglement router, are comprised of congruent photons and/or incongruent photons. The invention adds a plurality of additional filter stacks at each output port such that they invert the action of the first birefringent stack at the input port. Intermediate output photons from the invention is input to two ports of an additional PBS where they are spatially projected according to their frequencies and polarizations. Two congruent photons of an entangled photon pair exit as an entangled pair in one direction, while two incongruent photons exit as an entangled pair in the orthogonal direction. If one photon is congruent and the other photon incongruent, the photons remain entangled but are spectrally divided into orthogonal directions. The invention&#39;s birefringent spectral filter accepts specific input frequencies from the ITU optical C-band grid for proper operation.

REFERENCE TO PRIOR APPLICATIONS UNDER 35 U.S.C. §120

This patent application is a Continuation Application of and claims thepriority benefit of pending non-provisional application Ser. No.14/824,390, having been filed in the United States Patent and TrademarkOffice on Aug. 12, 2015 and now incorporated by reference herein.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government for governmental purposes.

BACKGROUND OF THE INVENTION

The present invention is germane to the propagation of polarizationentangled photon pairs collinearly incident upon a modifiedhyperspectral birefringent filter stage and routed intact into twopossible directions upon exit from the filter. Polarization entanglementis preserved.

Referring to FIG. 1, a previous similar effort, specifically thehyperspectral filter 30, was designed by Optical Physics Company andconstructed under an AFRL Small Business Innovative Research (SBIR)contract award, and was solely intended as a network “hub” residing in ageostationary or geosynchronous orbit. Its mission is to support lowprobability of detection, interception, and exploitation links at veryhigh data rates simultaneously to several users nearer to or on earthover a geographical region roughly the size of the Midwest UnitedStates. Such users will henceforth be referred to as “spokes.” A hub andspoke network configuration, electromagnetic beams transportinginformation between the spokes near earth and the hub at geo orbit arecomprised of many photons, say in the 1550 nm hand. They are classicalelectromagnetic beams, fully describable by classical electrodynamics.

As previously mentioned, an existing patented birefringent spectralfilter stage, U.S. Pat. No. 7,400,448, was awarded to Richard Hutchins,Optical Physics Company [1] and manifested in SBIR contractFA8750-11-C-0163. It is intended for securing free space lasercommunications applications where robust wide angle acceptance in bothazimuth and elevation is required. In U.S. Pat. No. 7,400,448, opticalsignals from the spokes are assigned distinct frequencies in the telecomband and possess common incident polarizations oriented 45° with respectto the optical axis of a set of birefringent spectral plates. Abirefringent filter stage 10 is comprised of the birefringent plates,called a Lyot filter or birefringent stack, (also referred to asbirefringent filter stack or filter stack) 10, followed by apolarization beam splitter 20, as shown in FIG. 1. The polarization beamsplitter's 20 plane of incidence is oriented 45° with respect to thebirefringent stack 10. It is therefore oriented in concert with theincident beams.

The primary contribution of U.S. Pat. No. 7,400,448 to OPC was to findan innovative way to increase the angular acceptance for simultaneous,multi-access laser communications employing wavelength divisionmultiplexing to distinguish distinct, spatially displaced users. U.S.Pat. No. 7,400,448 incorporates a broadband half-wave plate between twobirefringent plates whose extraordinary axes are perpendicular to oneanother and to the propagation direction of the laser light. Incomingfrequencies whose optical path difference phase over the Lyot filterstack is an even integral multiple of π will suffer no polarizationchange in transit through the filter. We call these frequenciescongruent. However, incoming frequencies whose optical path differencephase is an odd multiple of π will suffer polarization rotations by 90°.We call these frequencies incongruent. Incoming congruent andincongruent beams are split into orthogonal directions at a polarizationbeam splitter, or PBS. Thus, the hyperspectral Lyot filter stackprepares incoming beams for spatial separation by the PBS, accomplishingwavelength division multiplexing for classical laser beams. Thoseclassical laser beams initially possess common polarizations preset tobe 45° with respect to the filter reference frame. Their polarizationsare, however, coincident with the PBS frame which is oriented 45° withrespect to the Lyot filter frame of reference. After exiting the Lyotfilter stack, one set of beams, say the congruent set, possesspolarizations orthogonal to the PBS plane of incidence; they reflectfrom the splitting surface. Incongruent beams possess polarizationsparallel to the PBS plane of incidence and transmit through the splitterinterface.

Another feature of U.S. Pat. No. 7,400,448 is the incorporation ofmoveable wedges which can tune the optical thickness of the birefringentwedges to select desired transmission frequencies. This is the primaryapplication U.S. Pat. No. 8,427,769 to Raytheon employs in their Lyotfitter tuning device. U.S. Pat. No. 8,427,769 accomplishes a very finefrequency tuning of a laser beam by passing the beam multiple timesthrough the tunable aspect of the wedges deployed in one aspect of theU.S. Pat. No. 7,400,448 to OPC Lyot filter, where again, OPC's filter isnot critical to the U.S. Pat. No. 8,427,769 to Raytheon application. Itis simply one medium to accomplish one function the multi-pass Raytheonpatent employs to tune a laser beam to a desired frequency. The presentinvention could perhaps utilize this capability for future use, but isnot compatible with the present invention at this because the presentinvention instead switches the routing by adjusting the incomingfrequencies for a given static Lyot stack thickness, and thus achievingits goal of preserving the polarization entanglement.

Still referring to FIG. 1, distinct input frequencies from the distantspokes are either congruent or incongruent with respect to the filterstack 10. In traversing the filter stack 10, polarization states ofcongruent frequencies are not rotated, while polarization states ofincongruent frequencies are rotated by 90°. The two polarization statespossessed by each beam exiting a filter stack 10 are either transmittedor reflected at the splitting interface within the polarization beamsplitter 20. In other words the filter stack 10 prepares incoming beamsfor splitting into orthogonal directions by the polarization beamsplitter 20, thus enabling wavelength division multiplexing (WDM) forsimultaneous access between the network “hub” and its spatiallydistributed “spokes.” Moreover, if classical beams comprising a sum ofvertical and horizontal polarized states containing many photons of twodistinct frequencies are propagating along a line and entering thehyperspectral filter stage 30, one will measure both horizontal andvertical polarized states exiting the beam splitter at both outputports. The beams are a sum of electric field amplitudes comprising manyphotons. An entangled photon pair state is different; beams aredivisible, single photons are not. Yet a pair of single photonsgenerated by interactions at their source, such as four wave mixing inan optical fiber pumped by a sufficiently intense laser pulse, canpossess two possible polarization states. They are either bothhorizontally polarized along the same coordinate axis, or they are bothvertically polarized along an orthogonal axis. Both such possibilitiesare equally probable, but the state is unknown until measurement. Theirjoint probability quantum amplitude is expressed as a sum of productprobability amplitudes,

$\begin{matrix}{ {\Gamma( {f_{1};f_{2}} )} \rangle_{i\; n} = {\frac{1}{\sqrt{2}}( { {f_{1},H_{1},{P_{1};f_{2}},H_{2},P_{1}} \rangle +  {f_{1},V_{1},{P_{1};f_{2}},V_{2},P_{1}} \rangle} )}} & (1)\end{matrix}$Equation (1) expresses the input quantum state as a collinearv entangledphoton pair prior to entering the hyperspectral filter stage 30 at P₁.The term on the left of the plus sign is the probability amplitude thatthe joint state contains two photons, one with congruent frequency, f₁,and one with incongruent frequency, f₂. Both are horizontally polarized,and both are incident on port P₁ in FIG. 1. The term on the right of theplus sign is the probability amplitude that the joint state contains twophotons, one with congruent frequency, f₁, and one with incongruentfrequency, f₂. Both are vertically polarized, and both are collinearlyincident on port P₁ in FIG. 1. The probability of measuring the productstate on the left is the square of the coefficient multiplying it, hereequal to ½. Similarly, the probability of measuring the product state onthe right is ½.

Polarization measurement entails projection of the state onto detectorswherein the photon's energy is converted into an electrical signal; thephoton is destroyed, its energy converted to electricity. In such aprocess, measurement of horizontal polarization of one photonnecessitates horizontal polarization of the other. The measurements are100% correlated. Likewise, vertical polarization measured on one photonnecessitates vertical polarization on the other. Again, measurements are100% correlated. Either possible outcome of a polarization measurementon the two photons occurs at random and, as stated above, equallyprobable. In no case is horizontal polarization measured on one photonand vertical polarization measured on the other. These correlationsarise from conservation of energy and angular momentum at the source ofthe photons, and subsequent engineered assurance that both possibilitiesexist until measurement of the initial state, i.e., by entangling thetwo possibilities. Assigning logic bit 1 to horizontal polarization andlogic bit 0 to vertical polarization, polarization measurements on theentangled state generates a random bit stream, useful for cryptographicpurposes and quantum information processing applications. In otherwords, an entangled polarization state can be a carrier of randomnumbers which can be securely shared between two parties.

It is important to note that if the photons are split into two distinctdirections as a function of frequency, and their polarizations aremeasured in two non-orthogonal two dimensional bases oriented relativeto one another by 45°, ambiguity is imparted to the value of a logic bitwhen the two polarization bases are different. For example, in a quantumkey distribution protocol application where legitimate users sharecommon knowledge of which basis is used in every transmission betweenthem, security is enhanced under intercept and reseed attacks by aneavesdropper who does not share the common basis choice. Eavesdropperswill be wrong a discernible fraction of the time, alerting legitimateusers of their intrusion. This added security measure is not present inutilization of the frequency for secret key generation in the stategiven by equation (1).

Frequency measurements of the initial state in equation (1) are notrandom. They occur in either possible measurement as a deterministicpair. The frequency degree of freedom comprises just one two-dimensionalbasis, not the two, two dimensional bases of polarization. In otherwords, the frequency degree of freedom is not as intrinsically secure aspolarization degrees of freedom for random number generation utilized inquantum cryptography.

When congruent frequency f₁ and incongruent frequency f₂ are bothhorizontally polarized, transit through the birefringent stack 10 leavesthe congruent frequency polarization intact, but rotates the incongruentfrequency by 90°, to a vertical polarization state. Or, if the twophotons are vertically polarized, again, the congruent frequencyphoton's polarization state is left intact, remaining vertical, but theincongruent photon's polarization state is rotated from vertical tohorizontal polarization. The joint probability amplitude exiting thepolarization beam splitter 20, and thus the hyperspectral filter stage30 becomes,

$\begin{matrix}{ {\Gamma( {f_{1};f_{2}} )} \rangle_{out} = {\frac{1}{\sqrt{2}}( { {f_{1},H_{1},{P_{2};f_{2}},V_{2},P_{3}} \rangle +  {f_{1},V_{1},{P_{3};f_{2}},H_{2},P_{2}} \rangle} )}} & (2)\end{matrix}$Transiting the polarization beam splitter 20, the two photons aredirected into two orthogonal directions. The polarization beam splitter20 is a projection operator. Horizontal polarizations exit P₃ in FIG. 1and vertical polarizations exit P₂. Polarization measurements of aphoton exiting P₃ are no longer randomly distributed, nor arepolarization measurements exiting P₂. Thus, the polarization randomnessof the input state given in equation (1) is lost upon projection andsubsequent measurement. In terms of information utility, they behavelike the frequency behaves prior to entering the hyperspectral filterstage 30 i.e., like one two dimensional basis without randomness.Frequency measurements, however, are random. After transiting thehyperspectral filter stage 30, they can provide anti-correlated randombit streams at either port. If the output of P₂ is f₁, the output of P₃is f₂, and vice versa.

Useful polarization entanglement is lost and replaced by less usefulfrequency entanglement. The difference in utility is the fact thatpolarization entanglement allows for measuring in at least two, twodimensional bases, while frequency entanglement allows measurement inonly one two dimensional basis. For quantum key distribution purposes,two non-orthogonal two dimensional bases allow ambiguity in aneavesdropper's interpretation of the logical meaning of theirmeasurements. One two dimensional frequency basis does not allow suchambiguity to be imparted to an eavesdropper. It is therefore desirableto modify the single hyperspectral filter stage 30 to recover the moreintrinsically secure polarization entanglement.

OBJECTS AND SUMMARY OF THE INVENTION

Key to free space operation is the capability to accept light incidenton a telescope aperture in which the hyperspectral birefringent filteraccepts various angles of incidence in both azimuth and elevation. Aprior art design of reference [1] (see FIG. 1) ensures robust angularacceptance. Its frequency resolution can be varied, but is typicallydesigned to accommodate the ITU 200 GHz telecommunications grid. It canbe configured for greater or lesser frequency discrimination. Wavelengthtunable sources for classical communications are required as input tothe prior art design, and by extension to the present inventionpresented here entailing quantum communications.

In an embodiment of the present invention, an apparatus for routingpolarization entangled photon pairs, comprises a first polarization beamsplitter having a first input, a first output and a second output, wherethe first and second outputs are orthogonal to each other; a firstbirefringent filter stack coincident with the input of the firstpolarization beam splitter into which a entangled photon pair is input;a second birefringent filter stack coincident with the first output; athird birefringent filter stack coincident with the second output; asecond polarization beam splitter having a second input, a third input,a third output and a fourth output wherein the second and the thirdinputs are orthogonal to each other and where the third and fourthoutputs are orthogonal to each other; a first mirror coincident withfirst output oriented so as to direct photons from first output into thesecond input; and a second mirror coincident with the second outputoriented so as to direct photons from the second output into the thirdinput.

In other embodiment of the present invention, a method for creatingcorrelated polarization measurements of photons at distinct locations,comprises the steps of a first step of rotating the phase of thosephotons in a polarization entangled photon pair having incongruentfrequencies; directing the polarization entangled photon pair into afirst beam splitter; a first step of passing those photons having apolarization aligned with a first beam splitter and reflecting in anorthogonal direction those photons having a polarization not alignedwith the first beam splitter; a second step of rotating the phase ofthose photons having been passed having incongruent frequencies andthose photons having been orthogonally directed having incongruentfrequencies; directing the passed and the orthogonally directed photonsinto a second beam splitter; and a second step of passing those photonshaving a polarization aligned with the second beam splitter andreflecting in an orthogonal direction those photons having apolarization not aligned with the second beam splitter; and measuringthe polarization of said photons exiting said second polarization beamsplitter.

Briefly stated, the present invention, a photon entanglement router,comprises a modified birefringent spectral filter followed by apolarization beam splitter (PBS). Frequency degenerate or non-degenerateentangled photons, generated by a collinear laser source and incident onone input port of the photon entanglement router, are comprised ofcongruent photons and/or incongruent photons. The invention adds aplurality of additional filter stacks at each output port such that theyinvert the action of the first birefringent stack at the input port.Intermediate output photons from the invention is input to two ports ofan additional PBS where they are spatially projected according to theirfrequencies and polarizations. Two congruent photons of an entangledphoton pair exit as an entangled pair in one direction, while twoincongruent photons exit as an entangled pair in the orthogonaldirection. If one photon is congruent and the other photon incongruent,the photons remain entangled but are spectrally divided into orthogonaldirections. The invention's birefringent spectral filter acceptsspecific input frequencies from the ITU optical C-band grid for properoperation.

Referring to FIG. 2, the present invention considers non-classicalelectromagnetic propagation through a modified hyperspectral filterstage known herein as a photon entanglement router 40. Suchnon-classical photon propagation entails entangled photon pairscollinearly incident on birefringent filter stacks L₁, L₂, and L₃ 50,60, 70, modified to preserve and route polarization entanglement. Amajor difference between the non-classical incident energy and classicalelectromagnetic incident energy is this: the latter polarization statesare known with certainty, while the former two photon polarizationstates are unknown; they can be either horizontally polarized orvertically polarized with respect to the photon entanglement router's 40optical axes. Entanglement constrains the two photons' relation to oneanother in that they are either both horizontally polarized or bothvertically polarized and randomly so. The principal use of the presentinvention's birefringent filter stacks 50, 60, 70 is to preservepolarization correlations by recovering useful entanglement betweenphotons exiting the hyperspectral filter stage 40 and spatially directit for further quantum information processing operations.

Still referring to FIG. 2, the present invention improves upon the priorart hyperspectral filter stage (see 30, FIG. 1), and recoverspolarization entanglement in the sense that each possible polarizationin each photon exits the photon entanglement router 40 randomly and 100%correlated. For one photon frequency congruent and the otherincongruent, one photon exits into one direction, the other photon intoan orthogonal direction. Moreover, if both photons are congruent withrespect to the photon entanglement router 40 (see FIG. 3), or bothincongruent with respect to the photon entanglement router 40 (see FIG.4), the entire input entangled state can be routed into one direction orthe orthogonal direction respectively. Thus, controlling the frequencyor the birefringence of the birefringent filter stack 50, 60, 70, thepresent invention's hyperspectral filter stage 40 acts as a routingswitch, assuring polarization correlations remain intact and random viapreservation of their entangled joint probability amplitude.

The present invention considers non-classical electromagnetic photonpropagation through a modified hyperspectral filter stage referred to asa photon entanglement router 40 involving just two photons possessing aquantum mechanical coherence relation called polarization entanglement.In the absence of loss and decoherence, the present invention'shyperspectral filter stage ensures 100% correlation of polarizationmeasurements between the two photons exiting the photon entanglementrouter 40.

This invention also allows for just one photon of the polarizationentangled pair to be routed in one particular direction, while routingthe other photon of the polarization entangled pair into another,distinct direction. Reiterating, passively operating on a collinearinput entangled photon pair, the present invention's modifiedhyperspectral stage 40 allows for routing intact an entangled pair intoone or an orthogonal direction, or to distribute one photon of the pairinto one direction and the other into a distinct direction whilemaintaining the quantum entanglement between them.

Referring to FIG. 2, the present invention, a photon entanglement router40, comprises a modified birefringent filter stack L₁ 50 followed by afirst polarization beam splitter (PBS) 20. Frequency degenerate ornon-degenerate entangled photons, generated by a collinear laser sourceand incident on one input port of the photon entanglement router 40, arecomprised of congruent photons and/or incongruent photons. The inventionadds a plurality of additional birefringent filter stacks L₂ and L₃ 60,70 at each output port such that they invert the action of the firstbirefringent stack L₁ 50 at the input port. Then, the photons are inputto two ports of a second PBS 100 via mirrors 80, 90 where they arespatially projected according to their frequencies and polarizations.Two congruent photons of an entangled photon pair exit as an entangledpair in one direction, while two incongruent photons exit as anentangled pair in the orthogonal direction. If one photon is congruentand the other photon incongruent, the photons remain entangled but arespectrally divided into orthogonal directions. The birefringent filterstacks 50, 60, 70 accept specific input frequencies from the ITU opticalC-band grid for proper operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a prior art hyperspectral birefringent filter.

FIG. 2 is a diagram of the present invention photon entanglement routershowing the objects of the invention wherein one frequency is congruentand one frequency is incongruent with respect to the Lyot filter stacks,all of which here are identical.

FIG. 3 is a diagram of the present invention photon entanglement routershowing the objects of the invention wherein both frequencies arecongruent with respect to the Lyot filter stacks, all of which here areidentical.

FIG. 4 is a diagram of the present invention photon entanglement routershowing the objects of the invention wherein both frequencies areincongruent with respect to the Lyot filter stacks, all of which hereare identical.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention makes use of the foregoing aspect of the OPCfilter of U.S. Pat. No. 7,400,448, though is not dependent on thatparticular filter. Other birefringent media, including strictlyelectro-optic media, could accomplish the same thing. However, it ispreferable to utilize the OPC filters in the implementation of thepresent invention because they are largely passive, have wide angularacceptance in both horizontal and vertical dimensions, and haveaccompanying electro-optical media performing fine tuning to compensatefor temperature variations. A major difference between OPC's design andthe present invention is that the OPC configuration distributesclassical signals according to frequency into distinct end points, forhigh rate and simultaneous multi-access users. A typical input state isshown in the following equation (3.), a product state.|Ψ_(class)

_(in) =|f ₁ ,Σ,z,N;f ₂ ,Σ,z;N

  (3)

The semicolon designates the fact that there are two beams, one withcongruent frequency f₁, the other with incongruent frequency, f₂, andboth possessing huge numbers, N, of photons that are propagating alongthe z axis. Both have the same polarization, Σ, perpendicular to the PBSplane of incidence. Transiting the birefringent Lyot stack, then thePBS, the state is transformed into equation (4).|Ψ_(class)

_(out) =f ₁ ,Σ,x,N;f ₂ ,Π,z;N

  (4)

The output state has been split into two beams, a congruent one withfrequency f₁ propagating along the x-axis and possessing polarization Σ,and the other incongruent beam, with frequency f₂, whose polarizationhas been rotated to Π, parallel to the PBS plane of incidence. It ispropagating along the z-axis.

In contrast to the OPC or Raytheon designs, the design requirement forthe present invention must accommodate the possibility for either Σ or Πincident product states being randomly input into the birefringent Lyotstage. That composite state is a linear combination of product states.It is shown in equation (5).

$\begin{matrix}{ \Psi_{entang} \rangle_{i\; n} = {{\frac{1}{\sqrt{2}} {f_{1},\Sigma,z,{1;f_{2}},\Sigma,{z;1}} \rangle} + {\frac{1}{\sqrt{2}} {f_{1},\Pi,z,{1;f_{2}},\Pi,{z;1}} \rangle}}} & (5)\end{matrix}$The state of equation (5) contains just two photons, but with twopossible measurement Outcomes. The state on the left is the possibilitythat both photons are Σ polarized, while the state on the right is thepossibility that both are Π polarized. Transiting the hyperspectralbirefringent Lyot filter stage, the state is transformed into equation(6).

$\begin{matrix}{ \Psi_{entang} \rangle_{out} = {{\frac{1}{\sqrt{2}} {f_{1},\Sigma,x,{1;f_{2}},\Pi,{z;1}} \rangle} + {\frac{1}{\sqrt{2}} {f_{1},\Pi,z,{1;f_{2}},\Sigma,{x;1}} \rangle}}} & (6)\end{matrix}$In (6), note that the state on the left has one Σ polarized photonexiting along x, while the other Π polarized photon exits along z; so,too, in the state on the right. It also has a Σ polarized photon exitingalong x, while the other Π polarized photon exits along z. Thus therandomness associated with polarization has been lost. That randomnesshas been transferred to the frequency degree of freedom. Congruentfrequency exits out x on the left possibility, but exits out z on theright possibility. Conversely, the incongruent frequency exits out z onthe left possibility, but exits out x on the right possibility. Only onepossibility will be measured and while the polarization degree offreedom is not randomly distributed out both exits, the frequency degreeof freedom is. The latter entanglement is more useful because itpossesses randomness.

The problem with just frequency entanglement (and therefore theshortcomings of U.S. Pat. No. 7,400,448 to OPC and U.S. Pat. No.8,427,769 to Raytheon) is that it is a degree of freedom with just twobasis states, f₁ and f₂. These cannot change. Polarization, on the otherhand, can be rotated to produce as many two dimensional bases as onewishes. The most useful two dimensional bases are those two associatedwith the PBS frame of reference and the Lyot stack frame of reference.They are oriented 45° relative to one another. These two, twodimensional polarization bases allow maximum ambiguity to aneavesdropper attempting to sift information shared between twolegitimate users in a cryptographic application. Thus the presentinvention's design intent is to recover useful polarization entanglementpossessing its former measurement randomness.

The configuration of the present invention restores randomness to thepolarization measurement statistics by inserting two additionalbirefringent Lyot stacks into both paths exiting the OPC Lyot filterstage after the PBS. These Lyots stacks are identical to the initialLyot stack. They invert incongruent frequency polarizations but leavecongruent frequency polarizations intact. Thus, the initial polarizationstates of the two photons are recovered, but now path separation hasbeen performed for spatial distribution. Hence, when passed through oneadditional PBS, a congruent photon will go in one direction, while anincongruent photon will go in another direction. This is shown inequation (7).

$\begin{matrix}{ \Psi_{modified} \rangle_{out} = {{\frac{1}{\sqrt{2}} {f_{1},\Sigma,x,{1;f_{2}},\Sigma,{z;1}} \rangle} + {\frac{1}{\sqrt{2}} {f_{1},\Pi,x,{1;f_{2}},\Pi,{z;1}} \rangle}}} & (7)\end{matrix}$Equation (7) states that congruent frequencies always exit the secondPBS of the modified system along x, while incongruent frequencies alwaysexit the second PBS of the modified system along z. Frequencymeasurement statistics are no longer random. However, polarizationmeasurement statistics are now random. Either Σ or Π polarizations willexit along x or z randomly. Since polarizations can be rotated, two, twodimensional bases can be employed for cryptographic applications.

This is just one novel and non-obvious capability the presentinvention's entanglement preserving configuration accomplishes. Byselecting both input frequencies to be either congruent or incongruent,the present invention can send the entangled pair to two orthogonaldirections for further quantum processing.

Referring to FIG. 1, in transit through a polarization beam splitter(PBS) 20, polarization states are split into two orthogonal directionsaccording to whether their polarization is perpendicular or parallel tothe plane of incidence on the splitting surface within the PBS. A moresuitable designation for the polarization states before and after thePBS are the designations Σ and Π, for polarizations perpendicular to thePBS plane of incidence and parallel to the plane of incidencerespectively. Thus the entangled pair probability amplitudes, equations(1) and (2), read

$\begin{matrix}{{ {\Gamma( {f_{1};f_{2}} )} \rangle_{i\; n} = {\frac{1}{\sqrt{2}}( { {f_{1},\Sigma_{1},{P_{1};f_{2}},\Sigma_{2},P_{1}} \rangle +  {f_{1},\Pi_{1},{P_{1};f_{2}},\Pi_{2},P_{1}} \rangle} )}}{ {\Gamma( {f_{1};f_{2}} )} \rangle_{out} = {\frac{1}{\sqrt{2}}( { {f_{1},\Sigma_{1},{P_{2};f_{2}},\Pi_{2},P_{3}} \rangle +  {f_{1},\Pi_{1},{P_{3};f_{2}},\Sigma_{2},P_{2}} \rangle} )}}} & (8)\end{matrix}$

FIG. 1 illustrates equations 8, the propagation of polarization statesthrough a prior art hyperspectral filter stage 30. A legend on the rightidentities the symbols denoting congruency and incongruency of thestates. The frequencies are necessarily distinct. L denotes a Lyot stackand PBS denotes a polarization beam splitter. Circles denotepolarization states perpendicular to the PBS plane of incidence, whichis oriented at 45° with respect to the Lyot stack. They are denoted bythe symbol Σ. Arrows denote polarizations parallel to the PBS plane ofincidence. They are denoted by the symbol Π. Congruent frequencypolarization states, denoted by solid symbols, remain invariant, whileincongruent frequency states, denoted by dashed symbols, are rotated by90° in transit through a Lyot stack. These quantum operations on theinput states prepares them for projection at the PBS interface.

Referring to FIG. 2, the present invention improves upon the prior artthrough the addition of at least two more birefringent (Lyot) filterstacks L₂, L₃ 60, 70 to the existing hyperspectral filter stage 30 andthe addition of at least one more polarization beam splitter 100. Theseimprovements over the prior art serve to both route and preservepolarization entanglement in the following three ways that depend onchoice of input frequencies:

Adding additional birefringent filter stacks 60, 70, as in oneembodiment where one in each path after the photon entanglement router's40 polarization beam splitter 20 is added, leaves invariant congruentfrequency polarization states that pass through them. However, theadditional birefringent filter stacks 60, 70 invert incongruentfrequency polarization states by an additional 90° so that theirpolarizations are returned back to their original input state. Also,adding mirrors 80, 90 direct the photons into an additional polarizationbeam splitter 100 after the two additional birefringent filter stacks60, 70 as in one embodiment directs the congruent frequencies in onedirection (i.e., passed through unredirected) and the incongruentfrequencies in a spatially distinct direction (i.e., reflected in anorthogonal direction). In effect, this device breaks up the collinearinput entangled state into its respective qubit states, while retainingthe entanglement constraint expressed by the joint distributionexpressed in the lower equation of FIG. 2, which is the output state.

In FIG. 2, a measurement at P₆ will reveal either a congruent Σ state,or a congruent Π state. Similarly, a measurement at P₇ will reveal anincongruent Σ state or an incongruent Π state. When taken together,those measurements will reveal either a congruent Σ state and anincongruent Σ state, or a congruent Π state and an incongruent Π state.This is a manifestation of polarization entanglement; polarizationmeasurements at distinct locations are correlated.

In FIG. 3, where both entangled photon frequencies are congruent, butnot necessarily equal, the present invention directs the outputentangled pair into a spatial direction perpendicular to the inputdirection. Polarization measurements at P₆ will reveal either two Σpolarization states, or two Π polarization states.

In FIG. 4, where both entangled photon frequencies are incongruent, butnot necessarily equal, the present invention directs the outputentangled pair into a spatial direction parallel to the input direction.Polarization measurements at P₇ will reveal either two Σ polarizationstates, or two Π polarization states.

In all cases, initial polarization entanglement is preserved at theoutput. A measurement of horizontal polarization on one photonnecessitates horizontal polarization on the other. Likewise, verticalpolarization measured on one photon necessitates vertical polarizationon the other. In no case is horizontal polarization measured on onephoton and vertical polarization measured on the other.

What is claimed is:
 1. A method for creating correlated polarizationmeasurements of photons at distinct locations, comprising steps ofbirefringent filtering those photons in a polarization entangled photonpair having incongruent frequencies so as to rotate the polarizationstate thereof; directing said polarization entangled photon pair into afirst beam splitter; beam splitting said polarization entangled photonpair so as to pass those photons having a polarization aligned with afirst beam splitter and reflecting in an orthogonal direction thosephotons having a polarization not aligned with said first beam splitter;birefringent filtering: those photons having been passed havingincongruent frequencies; and those photons having been orthogonallydirected having incongruent frequencies so as to invert the polarizationstate rotation imparted by birefringent filtering; reflecting saidpassed and said orthogonally directed photons into a second beamsplitter; beam splitting said reflected photons so as to pass thosephotons having a polarization aligned with said second beam splitter andreflecting in an orthogonal direction those photons having apolarization not aligned with said second beam splitter; and measuringthe polarization of said photons exiting said second beam splitter. 2.The method of claim 1, said steps of birefringent filtering compriserotating said polarization state by 90 degrees.
 3. The method of claim1, said step of reflecting comprises redirecting by impingement on amirrored surface.
 4. The method of claim 1, said steps of birefringentfiltering possess equivalent birefringence.
 5. The method of claim 1,said step of birefringent filtering a congruent photon and anincongruent photon pair at a polarization perpendicular to said firstbeam splitter's plane of incidence will result in said step of beamsplitting producing: the congruent photon perpendicular to said firstpolarization beam splitter's plane of incidence, and the incongruentphoton perpendicular to said first polarization beam splitter plane ofincidence.
 6. The method of claim 1, said step of birefringent filteringa congruent photon and a congruent photon pair at a polarizationparallel to said first beam splitter's plane of incidence will result insaid step of beam splitting producing: the congruent photon parallel tosaid first polarization beam splitter's plane of incidence, and theincongruent photon parallel to said first polarization beam splitter'splane of incidence.
 7. The method of claim 1, said step of birefringentfiltering a first congruent photon and a second congruent photon pair ata polarization perpendicular to said first beam splitter's plane ofincidence will result in said step of beam splitting producing: thefirst congruent photon perpendicular to said first polarization beamsplitter's plane of incidence, and the second congruent photonperpendicular to said first polarization beam splitter's plane ofincidence.
 8. The method of claim 1, said step of birefringent filteringa first congruent photon and a second congruent photon pair at apolarization parallel to said first beam splitter's plane of incidencewill result in said step of beam splitting producing: the firstcongruent photon parallel to said first polarization beam splitter'splane of incidence, and the second congruent photon parallel to saidfirst polarization beam splitter's plane of incidence.
 9. The method ofclaim 1, said step of birefringent filtering an first incongruent photonand an second incongruent photon pair at a polarization perpendicular tosaid first beam splitter's plane of incidence will result in said stepof beam splitting producing: the first incongruent photon perpendicularto said first polarization beam splitter's plane of incidence, and thesecond incongruent photon perpendicular to said first polarization beamsplitter's plane of incidence.
 10. The method of claim 1, said step ofbirefringent filtering an first incongruent photon and an secondincongruent photon pair at a polarization parallel to said first beamsplitter's plane of incidence will result in said step of beam splittingproducing: the first incongruent photon parallel to said firstpolarization beam splitter's plane of incidence, and the secondincongruent photon parallel to said first polarization beam splitter'splane of incidence.